The present invention relates to magnetic polishing fluids useful for polishing metallic surfaces, particularly metal surfaces in the microelectronic industry.
Chemical-mechanical polishing is a commonly used technique for planarization of bare silicon, silica and other ceramic wafers. In chemical-mechanical polishing, the wafers are held against a rotating polishing pad wetted by a slurry consisting of colloidal abrasive particles (silicon dioxide, alumina, cerium oxide, etc.) and specific additives to enhance the rate and quality of polishing. When silica is used, a high pH is required. An oxide or hydroxide surface layer forms on the wafer and the reaction products are removed by mechanical action of the pad and the colloidal silica slurry. Similar processes occur when other abrasives are used.
Copper interconnects in ultra-large scale integrated (ULSI) circuit layouts were introduced relatively recently into the semiconductor industry. They became possible as a result of improvements in chemical-mechanical planarization (CMP) technology developed during the last two decades. The adoption of CMP for polishing or removing thin metal layers, such as copper, aluminum, tungsten, titanium and tantalum layers, has enabled their commercial use and markedly changed semiconductor processing. The CMP techniques presently used for metals are very similar to those used for polishing ceramics described above.
The use of CMP has led to a large increase in the amount of metal being polished in the semiconductor industry. Only ten years ago, the polishing of dielectric ceramics constituted more than 90% of semiconductor materials being polished. The current volume of metal being polished in the industry exceeds by several hundred percent that polished formerly.
The bending and stresses of the rotating polishing pad used in convential CMP polishing, strongly influence the quality of planarization. The adverse effects of a polishing pad can be avoided, or at least mitigated, by using a magnetic polishing fluid which forms a plasticized mass in an a inhomogeneous magnetic field.
Magnetorheological fluids (MRFs) for polishing ceramics and optical materials are known in the art. With MRFs, the polishing pad common to conventional polishing is dispensed with entirely.
The viscosity of magnetorheological fluids increases in a magnetic field and the fluids, which contain magnetic particles, acquire the characteristics of a plasticized solid, that is one exhibiting Bingham properties, forming an abrasive mass with a viscosity and elasticity suitable for polishing.
Typical MRF formulations are discussed in U.S. Pat. Nos. 5,804,095, 5,795,212, 5,525,249, 5,449,313, and 4,821,466. These references when mentioning polishing, do so only in the context of polishing ceramic or optical materials. They do not discuss polishing metal surfaces.
Polishing machines used with prior art MRF compositions causes them to acquire viscous, plasticized properties known as Bingham properties, when under the influence of magnetic forces. When in a magnetic field, the MRF is hard enough to be used as a polishing tool. However, these polishing fluids reach their fully developed Bingham state only at the outset of the polishing process. Once the substrate being polished begins to move relative to the MRF, the shear stress of the abrasion process causes the MRF to lose its Bingham properties and its plasticized characteristics. The MRF remains viscous throughout the process but returns to a liquid, non-plasticized state almost immediately after abrasion begins, reducing polishing efficiency.
The magnetorheological fluids in the above-cited compositions usually provide a mechanical mechanism for the polishing process. In addition, these fluids generally contain magnetic and/or abrasive particles with sizes in excess of 1 xcexcm. These large size particles, and especially their agglomerates, introduce defects, scratches and non-uniformities onto the surface being treated.
Finally, because of the large size magnetic or abrasive particles used, these fluids have a tendency to settle due to gravitational forces. Because of their size, the abrasive particles act as agglomeration centers, contributing to the aggregation of magnetic particles and to the instability of the magnetic fluid. Any use of such fluids requires prior agitation.
The present invention relates to magnetic polishing fluid compositions suitable for polishing metal surfaces. In particular, it is suitable for polishing metal interconnects, vias and layers used extensively in current ULSI manufacturing.
One embodiment of the present invention teaches a magnetic polishing fluid composition comprising colloidal size magnetic particles with a mean particle size range from about 0.01 xcexcm to about 0.15 xcexcm, colloidal size polishing particles with a mean particle size range from about 0.01 xcexcm to about 0.1 xcexcm, at least one stabilizer for stabilizing the colloidal size particles, at least one oxidizer, at least one inhibitor for controlling the rate of oxidation of the oxidizer, and a carrying fluid where the pH of the composition is in the range from between about 0.5 to about 5.5.
In another embodiment based on the above, the magnetic polishing fluid composition does not include the colloidal polishing particles. In yet another embodiment based on the above, the polishing fluid does not contain an inhibitor.
A method for preparing the above compositions includes the steps of preparing a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid, preparing a colloidal size polishing particle dispersion in a carrying fluid, adding to the colloidal size polishing particle dispersion at least one oxidizer and at least one inhibitor, mixing said colloidal size magnetic particle dispersion and said colloidal size polishing particle dispersion to form a mixture, and adjusting the mixture to a final pH of about 0.5-5.5, if not already at the pH.
In yet another embodiment, a multi-phase magnetic composition, a magnetic polishing fluid composition contains both colloidal and non-colloidal magnetic particles. The non- colloidal magnetic particles have a mean particle size range from about 0.15 xcexcm to about 3.0 xcexcm and the colloidal magnetic particles have a mean particle size range from about 0.01 xcexcm to about 0.15 xcexcm. The composition also contains at least one oxidizer, at least one inhibitor for inhibiting the rate of oxidation of the oxidizer, at least one stabilizer for stabilizing said colloidal particles, an additive for adjusting viscosity, and a carrying fluid. The pH of the final composition is in a range from about 0.5 to about 5.5.
Yet another embodiment based on the embodiment discussed immediately above, does not include an inhibitor. A second multi-phase embodiment does not require colloidal polishing particles.
The invention also teaches a method for producing the above composition comprising the steps of preparing a non-colloidal size magnetic particle dispersion by adding the particles to a mixture of a viscosity additive in a carrying fluid. This is followed by preparation of a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid. This is further followed by preparing a colloidal size polishing particle dispersion in a carrying fluid and adding to the colloidal size polishing particle dispersion at least one oxidizer and at least one inhibitor. The colloidal size magnetic particle dispersion and the colloidal size polishing particle dispersion are mixed to form an intermediate mixture, which is then mixed with the non-colloidal size magnetic particle dispersion. Finally, the pH of the composition is adjusted to a pH in the range of about 0.5 to about 5.5, if the composition is not already at the proper pH in that range.
In yet another embodiment, a magnetic polishing fluid composition is taught which comprises magnetic particles having a mean particle size range from about 0.15 to about 3.0 xcexcm, colloidal size polishing particles having a mean particle size range from about 0.01 to about 0.1 xcexcm, at least one oxidizer, at least one inhibitor for inhibiting the rate of oxidation by the oxidizer, at least one stabilizer for stabilizing the colloidal particles, an additive for adjusting viscosity, and a carrying fluid where the pH of the composition is from about 0.5 to about 5.5
Various specific compositions are taught by the present invention. One such composition includes colloidal size magnetic particles comprised of colloidal size magnetite (Fe3O4) particles. These particles have a weight percent content of the total composition ranging from about 20 to about 55 wt %. Colloidal size polishing particles comprising colloidal size alumina (Al2O3) particles have a weight percent content in the total composition ranging from about 1 to about 20 wt %. Other components in the composition include an oxidizer comprising Fe(NO3)3 having a weight percent content ranging from about 1 to about 10 wt. %, an inhibitor comprising K2Cr2O7 having a weight percent content in said composition ranging from about 0.01 to about 2 wt. %, a stabilizer comprising perchloric acid having a weight percent content ranging from about 0.5 to about 20 wt. %, and a carrier fluid adjusted to a pH in the range of about 0.5 to about 5.5 comprising any remaining wt % of said composition.
Another composition taught by the invention includes non-colloidal size magnetic particles selected from carbonyl iron or magnetite particles. These particles have a weight percent content in the final composition of from about 21 to about 38.5 wt %. Colloidal size magnetic particles comprising colloidal size magnetite particles having a weight percent content in said composition from about 3 to about 7.5 wt %, and colloidal size polishing particles comprising colloidal size alumina particles having a weight percent content in said composition from about 1 to about 10 wt % are two other components of the system. A stabilizer comprising perchloric acid has a weight percent content in the composition from about 0.5 to about 10 wt. %, an oxidizer comprising ferric nitrate has a weight percent content in said composition ranging from about 1 to about 10 wt %, an inhibitor comprising potassium dichromate has a weight percent content in the composition ranging from about 0.01 to about 2 wt %, a viscosity adjusting additive comprising polyethylene glycol has a weight percent content in the final composition from about 0.1 to about 5 wt %, and a carrier fluid, adjusted to a pH in the range from about 0.5 to about 5.5, comprises any remaining wt % of the final composition.
A third composition taught includes non-colloidal magnetite particles having a weight percent content in the final composition ranging from about 20 to about 55 wt %, colloidal size polishing particles comprising colloidal size alumina particles having a weight percent content in the composition ranging from about 1 to about 20 wt %, a stabilizer comprising perchloric acid having a weight percent content in the composition ranging from about 0.5 to about 20 wt. %, a viscosity additive comprising polyethylene glycol having a weight percent content in the composition ranging from about 0.1 to about 10 wt. %, an oxidizer comprising ferric nitrate having a weight percent content ranging from about 1 to about 10 wt. %, an inhibitor comprising potassium dichromate having a weight percent content ranging from about 0.01 to about 2 wt. %, and a carrier fluid, adjusted to a pH in the range from about 0.5 to about 5.5, comprises any remaining wt % of said composition.
The present invention relates to various magnetic polishing fluid (MPF) compositions that can be used to polish metal substrates, and even some ceramic-like materials. Among the metals that can be polished are copper, aluminum, tungsten, titanium, and tantalum and among the ceramics which can be polished are TiN and TaN. The compositions are particularly advantageous as MPFs for polishing copper interconnects in the semiconductor industry. They can also polish the dielectric layers often present in semiconductor multi-layers. By varying the details of the formulation of these MPFs, the rate and quality of polishing can be controlled. The magnetic polishing fluids of the present invention polish at lower viscosities than the MRFs cited above, thereby producing better quality surfaces.
In the present invention an oxidizer is used to create a metal oxide on the surface of a metal. In general metal oxides are softer and therefore more easily polished then pure metals. The rate of polishing depends on the concentration of oxidizer present. Usually, the inhibitor is added to control the rate of polishing by slowing the rate of oxidation of the metal surface.
A magnetic polishing fluid (MPF) in a magnetic field, although forming a plasticized mass when the field is activated, retains or reverts to the properties of a liquid during the polishing process. When in its plasticized state the mass possesses Bingham properties.
A magnetic polishing fluid comprising magnetic particles, responds to a magnetic field gradient by increasing its viscosity and changing into a plasticized mass. Any polishing particles contained in the fluid are trapped in the plasticized mass and pushed out of the fluid in a direction perpendicular to the gradient.
This mass is an effective polishing tool when it comes into contact with a moving work surface. Upon contacting the polishing surface, and under the shear stress of the abrasion process, the plasticized mass reverts back to an almost liquid state.
In addition, when a polishing solution contains magnetic particles, particularly magnetic particles of non-colloidal size, the particles interact and form dendritic structures when plasticized under the influence of magnetic fields. These plasticized structures contain pores. A liquid media containing a polishing slurry comprising colloidal abrasive particles can fill the pores in the plasticized structure. The polishing rate is enhanced when the pores in the plasticized polishing mass are filled in this manner. The liquid containing the abrasive particles rises to the surface of the plasticized mass through capillary action. At the surface, the polishing particles are pressed by the polishing machine against the work surface.
The magnetic polishing fluids of the present invention usually comprise at least one type of magnetic particles, colloidal size abrasive particles, at least one oxidizer, at least one inhibitor, at least one stabilizer, and a carrying fluid. In one embodiment, the magnetic particles are of colloidal size and first dispersed in a stabilizer in a carrying fluid before mixing with colloidal abrasive particles. In another embodiment, two sizes of magnetic particles are used, one colloidal and one non-colloidal. Each size of magnetic particle is dispersed separately in its own carrying fluid before the two dispersions are mixed. Usually, inhibitors and oxidizers are added to the colloidal polishing particle dispersion. The oxidizer can however be kept separate and added to the solution only immediately prior to polishing.
For purposes of the present invention, colloidal size magnetic particles are deemed to be particles with mean particle sizes of about 0.15 xcexcm or less, while colloidal size polishing particles are deemed to be particles with mean particle sizes of about 0.1 xcexcm or less.
Certain embodiments of the invention may require a viscosity additive to control viscosity and/or a pH additive to adjust pH. Additives to increase the rate and enhance the quality of the polishing process can be added to the composition, as needed. In all the embodiments discussed, there is little agglomeration or sedimentation in the fluid over a period of one year at room temperature. The composition will be ineffective if the oxidizer is not kept separate from the rest of the composition when the composition is stored for long periods of time.
The magnetic particles used in these compositions can be chosen from many different ferrous and non-ferrous magnetic materials. These include, but are not limited to, ferromagnetic metals such as Fe, Ni, and Co, their alloys, ferrites, chromium oxides such as CrO2, iron oxides such as Fe3O4, iron phosphides, and cobalt oxides such as Co3O4. Carbonyl iron or magnetite (Fe3O4), particularly the latter, are the preferred sources for non-colloidal size magnetic particles. The preferred mean particle size range for non-colloidal magnetic particles is generally about 0.15-3.0 xcexcm, and even more preferably 0.5-1.0 xcexcm.
Capillary action is needed to transport the liquid containing the abrasive colloidal particles to the surface of the plasticized magnetic particle mass. The magnetic particles form quasi-columns with the pathways between them thought of as capillary tubes. The average diameter of the capillary is determined by magnetic particle size. The capillary must be narrow enough to generate a large capillary force but not so narrow as to be clogged by abrasive particles. A rough calculation based on these principles, assuming abrasive particles with diameters of about 0.1 micron, can establish 3 microns as an approximate upper bound for the magnetic particles that should be used.
Colloidal magnetite, the preparation of which is described below, is the preferred source of colloidal magnetic particles. The particles are formed from a mixed ferrous-ferric chloride solution to which either ammonium hydroxide, NaOH or urea is added. Methods for preparing colloidal magnetite other than the one disclosed herein below can also be used, such as the method of Massarat (IEEE Transactions of Magnetics, Vol. Mag 17 (#2) March 1981). Colloidal magnetic dispersions are also commercially available from manufactures such as Ferrofluids, Inc. The preferred mean particle size range for colloidal magnetic particles is generally about 0.01-0.15 xcexcm, preferably 0.03-0.05 xcexcm.
Colloidal size polishing particles can be chosen from many types of oxides, such as xcex1-Fe2O3, Al2O3, MgO, MnO2 or SiO2. Colloidal size particles of carbides e.g. SiC, nitrides e.g. Si3N4 and polycrystalline diamond can also be used as abrasives. The colloidal polishing particles, preferably alumina (Al2O3), generally have a mean particle size range of about 0.01-0.1 xcexcm. An even more preferred mean particle size range for the polishing particles is 0.03-0.05 xcexcm. In the case of diamond, nanometer size particles can be used.
The metal polishing mixtures also include an oxidizer selected from the group consisting of Fe(NO3)3, H2O2, KIO3, and K3[Fe(CN)6] with the first two being most preferable. The fluid usually includes an inhibitor selected from the group consisting of toluidine, xcex2-naphthylamine, K2CrO4, K2Cr2O7, K2MoO4, and benzotriazol, preferably at least one of the last three. The oxidizer is needed for producing a thin oxide film on the metal substrate and the inhibitor is required for passivation of the metal surface, thereby controlling the rate of polishing. The exact choice of additives and inhibitors depends on the metal being polished.
A stabilizing additive for the polishing agent can be chosen from many different non-ionic, anionic and cationic stabilizers. In a preferred embodiment of the invention the stabilizing agent is perchloric acid (HCIO4), which also helps adjust pH. Other stabilizers which can be used are Fe(NO3)3, polyethylene (10) isooctylphenyl, polydispersed nonylphenol, sodium polyethoxylate, sodium dodecylsulfonate, decylmethylsulfoxide and sodium lignosulfonates. The same stabilizer is usually, but not necessarily, used to stabilize both the colloidal size magnetic and colloidal size polishing particles. Use of low molecular weight stabilizers is preferred since high molecular weight stabilizers, and in particular polymeric surfactants, decrease the rate of polishing, sometimes by as much as a factor of four or five.
All of the magnetic fluids require a carrying fluid. The preferred carrier fluid is usually water, and even more preferably de-ionized water. Water is particularly advantageous when organic residues would be problematic as is often the case is in the microelectronic industry.
Other carrying fluids can also be used. These include, but are not limited to, polar organic solvents, such as alcohols and ketones, as well as mixtures of water and polar organic solvents.
In some embodiments, especially those using non-colloidal magnetic particles, viscosity additives are required. These can be chosen from water soluble polar polymeric materials, such as polyalcohols having molecular weights (MW) of about 300,000 to about 1,000,000. A preferable viscosity additive is polyethylene glycol (PEG). A typical static viscosity, that is the viscosity of the composition when not being used for processing, is about 300-1200 cps.
The pH of the magnetic fluid composition should be from about 0.5 to about 5.5, preferably 1.5-4.5. The exact pH of a composition depends on its intended use. Tungsten can be polished with compositions having pHs of about 4.1-4.2, while polishing aluminum requires compositions with pHs of about 1.9-2.0. The pH is adjusted using organic and inorganic acids such as acetic acids, HCIO4, H2SO4, HNO3 and HCl, with the last three being most preferred.
The MPFs of the present invention can be prepared in several ways, the exact method of preparation depending on the composition of the fluid. A preferable method for preparing the MPFs of this invention is a multi-step process.
When non-colloidal size magnetic particles are used, a dispersion of such particles in a carrying fluid is prepared. The particles are usually dispersed in the presence of a viscosity additive, after the additive has been mixed into the carrier fluid. When the carrier fluid itself is a multi-component system, the carrier fluid is usually prepared from its components prior to adding the viscosity additive.
The method for preparing a colloidal magnetic particle dispersion is similar to that of the non-colloidal one discussed immediately above except that a stabilizer replaces the viscosity additive. The colloidal magnetic powder can be purchased or synthesized. When the colloidal powder is synthesized, the stabilizing agent is usually added directly to the powder immediately after synthesis. Colloidal size iron particles can be synthesized as described in Example II below or by other methods described in the literature.
A dispersion of stabilized colloidal size polishing particles in a carrying fluid is then prepared. The stabilized colloidal dispersion can be prepared by any known method. Commercially available colloidal polishing particle dispersions can also be used. After the colloidal polishing solution is prepared, at least one oxidizer and at least one inhibitor is added.
After the colloidal polishing particle dispersion and either the non-colloidal magnetic particle or the colloidal magnetic particle dispersion have been prepared, the magnetic particle dispersion and the polishing dispersion are mixed slowly with vigorous stirring. Finally, the pH of the mixture is adjusted to a value from about 0.5 to about 5.5, preferably 1.5-4.5, if the mixture is not already at a pH in that range.
The magnetic particle, polishing particle and stabilizer contents in the final MPF composition range from about 20 to 55, from about 1 to 20, and from about 0.5 to 20 wt. %, preferably from about 25 to 35, from about 3 to 15, and from about 1 to about 8 wt. %, respectively. When a viscosity additive is required, it is present in the final MPF composition in a wt % varying from about 0.1 to about 10 wt %, preferably from about 1 to about 5 wt %. The concentrations of the oxidizer and inhibitor are from about 1 to about 10 wt % and from about 0.01 to about 2 wt %, preferably from about 2 to about 7 wt % and from about 0.03 to about 0.9 wt % %, respectively. Generally, the larger the wt % of stabilizer used, the slower the rate of polishing. It is to be understood that here and elsewhere in the specification when weight percents do not total 100%, the remainder is made up of carrying fluid, adjusted to the proper pH as described above.
A multi-phase magnetic polishing fluid can be prepared as follows. Separate dispersions of non-colloidal magnetic particles, colloidal magnetic particles and colloidal polishing particles are prepared as described above. At least one oxidizer and at least one inhibitor are added directly to the colloidal polishing dispersion. After the three dispersions have been prepared, the stabilized colloidal polishing particle dispersion is slowly added to, and mixed vigorously with, the stabilized colloidal magnetic particle dispersion to form an intermediate dispersion. The intermediate dispersion is gradually mixed with the non-colloidal magnetic particle dispersion, while stirring vigorously. Finally, the pH of the mixture is adjusted to a value from about 0.5 to about 5.5, preferably 1.5-4.5, if the mixture is not already at a pH in that range.
The proportions of non-colloidal magnetic, colloidal magnetic and colloidal polishing particles in the final MPF composition range between 21-38.5, 3-7.5 and 1-10 wt %, preferably 25-28, 3.75-5.25 and 3-5 wt %, respectively. The proportions of the stabilizer and viscosity additive range from about 0.5 to 10 wt % and fromabout 0.1 to 5 wt. %, preferably from about 0.5 to 5 wt % and from about 1 to 5 wt. %, respectively. The concentrations of the oxidizer and inhibitor range from about 1 to 10 wt % and from about 0.01 to 2 wt %, preferably from about 2 to 7 wt % and from about 0.03 to 1.2 wt %, respectively. In general, the ratio of non-colloidal to colloidal magnetic particles should be about from about 4.9:1 to about 6.1:1, preferably from about 5.3:1 to about 5.7:1.
In the multi-phase embodiment, a method of preparation was described wherein the colloidal polishing particle dispersion is first mixed with a colloidal magnetic particle dispersion. The resulting intermediate dispersion is then further mixed with the non-colloidal magnetic particle dispersion. An alternate, but equally acceptable, procedure would be to first mix the abrasive and non-colloidal magnetic dispersions to form an intermediate dispersion which in turn would be mixed with the colloidal magnetic dispersion.
While in previous embodiments, the oxidizers are added during the preparation of the polishing fluid, in other embodiments the oxidizer can be added to the other already mixed components immediately prior to the final composition""s use. This extends the shelf-life of the MPF.
In previously described embodiments, a colloidal polishing solution is always part of the MPF. Other MPF embodiments, suitable for certain purposes, do not have to contain a colloidal polishing agent. Satisfactory polishing results can be obtained in certain situations.
Finally, while in previously described embodiments an inhibitor is present, this is not always necessary. It has been found that reasonable polishing results may be obtained for certain purposes and under certain conditions without an inhibitor being present in the MPF.
The polishing fluids of the present invention can be used with most of the magnetic polishing machines discussed in the literature. Typical magnetic polishing machines are described in U.S. Pat. Nos. 4,821,466, 5,449,313, 5,577,948, 5,616,066, and 5,839,944. In addition, the compositions of the present invention were tested using a polishing machine that employs a series of intermittent impacts from a plasticized quasi-solid MPF.
The polishing fluids described in this Application can be used to polish metal interconnects, plugs and layers for the microelectronics industry, metallic tubing, and other metal surfaces.
The following are several magnetic polishing fluid compositions prepared using the procedures described above.